1. Field of the Invention
The present invention relates to a process for desalination or de-salting of seawater and more specifically by use of hydrophobic porous membranes in conjunction with non laminar flow.
2. Background Information
The need for the separation of fresh water from seawater and numerous processes and apparatus for accomplishing this task are well known. The separation of water vapor and its condensation into water by means of hydrophobic membrane are also known.
One example of an apparatus for carrying out such a process is disclosed in U.S. Pat. No. 4,545,862 to Gore. In Gore, a membrane distillation module is provided with a thin flexible microporous membrane positioned against an impermeable condenser sheet that is stiffer in bending than the membrane. Distillate vapor diffuses through the membrane from the hot feed flowing rapidly past the membrane, and condenses and collects there between, and in intimate contact with the membrane and the condenser sheet. The collector sheet is thin such as on the order of 0.001 to 0.005 inches. The collector sheet is positioned adjacent to the membrane such that condensing vapor is separated from the heated seawater when distillate begins to form. Cooling fluid is provided on the side of the impermeable membrane opposite to the collection space between the membrane and the collector. As sea water flows by the thin membrane, and undulating force tends to move the distillate toward an outlet. Another similar form of distillation apparatus using a hydrophobic porous membrane is shown in U.S. Pat. No. 4,818,345 to Jonsson. Jonsson also uses a hydrophobic membrane and a condenser sheet with a defined air gap between membrane and condenser. A flow of air through the air gap acts to strip the distillate from the air gap volume. The air gap between membrane and condenser is in the range of 0.2 mm to 1.0 mm. Another embodiment of distillate chamber type of desalination device is illustrated in U.S. Pat. No. 4,718,985 to Kjellander. The apparatus of Kjellander is characterized in that the membrane through which the saltwater flows, the condenser surface, and the chamber through which cooling water flows are all coaxial. Consequently the collection chamber or air gap volume is also coaxial. Again an air flow through the collection chamber removes the distillate. In each of the above patents, there is a common element of a defined space between a vapor permeable membrane and a condenser wall separating the distillate from a cooling fluid. As pointed out in Gore, a thin air gap between membrane and condenser tends to limit the rate at which distillate can be extracted and a large air gap between membrane and condenser would tend to limit productivity.
There is a need for a membrane distillation process that will provide for rapid removal of distillate without limiting productivity.